1. Field of the Invention
The present invention relates to an optical recording medium used for recording and storage of information.
2. Description of the Related Art
Conventionally, Compact Disk-Write Once type (hereinafter referred to as CD-WO) optical recording media obtained by laminating on a substrate a recording layer (optical absorption layer), an optical reflection layer and a protective layer have been known in Japanese Laid-Open Patent 183296 (1983), 46019 (1978), 37851 (1983) and 159842 (1989).
However, copper phthalocyanine (Trade mark; OLEOSOLE FAST BLUE EL, manufactured by SUMITOMO CHEMICAL CO., LTD.) which constitutes the recording layer (optical absorption layer) in the optical recording medium disclosed in Japanese Laid-Open Patent 183296 (1983) could not provide sensitivity required for the optical recording media.
The medium of Example 2 in Japanese Laid-Open Patent 46019 (1978) was poor in sensitivity and durability, and did not satisfy reflectance of 65% or more in a wavelength of 780 to 830 nm, which condition is required for the optical recording media, CD-WO in particular.
Japanese Laid-Open Patent 37851 (1983) disclosed the same constitution as that of the present invention. However, the reflectance of optical recording media obtained in the examples was 30 to 51% and lower than the reflectance of 65% which was required for CD-WO. Durability was also unsatisfactory.
As illustrated in Japanese Laid-Open Patent 159842 (12989), the medium prepared by using a common cyanine colorant described in, for example, Photosensitive Dye Table published by Japan Photosensitive Dye Research Institute (1969), led to insufficient sensitivity or reflectance lower than 65%.
The cyanine colorant disclosed in the examples of EP 353,394 and the phthalocyanine colorant disclosed in the examples of EP 353,393 had poor sensitivity and signal property, and were unsuitable as the colorant for the CD-WO type optical recording media.
3. Summary of the Invention
The object of the invention is to provide a novel CD-WO type optical recording medium having reflectance of 65% or more which has not been obtained in abovementioned conventional examples.
As a result of an intensive investigation in order to accomplish the above object, the present inventors have found that, when the recording layer of the CD-WO type optical recording medium contains a near infrared absorber having a melting point of from 150.degree. to 300.degree. C. and a decomposition initiating temperature of from 200.degree. to 350.degree. C., the optical recording medium constituted of the recording layer containing the near infrared absorber has high sensitivity for writing signals, good symmetry of written signals and small distortion in recording.
The present inventors have further found that, when the recording layer contains a near infrared absorber having a maximum absorption wavelength (.lambda. max) of from 670 to 720 nm using carbon tetrachloride, chloroform or toluene as a solvent, the medium can maintain its reflectance at 65% or more. Thus the present invention has been completed. With respect to the melting point and decomposition initiating temperature of the near infrared absorber used in particular, introduction of halogen atoms or specific substituents has also been found to be effective for achieving the object.
The present inventors have found that particularly the melting point and the decomposition initiating temperature have great contribution to the sensitivity in recording and stability in regeneration, and completed the invention on the basis of the discovery.
The optical recording media of the invention have a constitution obtained by successively laminating the recording layer, a metal reflection layer and a protective layer on a transparent substrate. The above recording layer is characterized by containing the near infrared absorber composed of a compound or a mixture of the same having a melting point in the range of from 150.degree. to 300.degree. C. and a decomposition initiating temperature in the range of from 200.degree. to 350.degree. C.
The melting point was measured with a common melting point tester. The decomposition initiating temperature was measured with a differential thermal-thermogravimetric simultaneous analyzer (DTA-TGA). The temperature of 5% decomposition was defined as the decomposition initiating temperature.